Melt butter in heavy large pot over medium heat. Add leeks and garlic; sauté until tender, about 6 minutes. Add potatoes and zucchini; sauté 5 minutes. Add 4 1/2 cups broth and bring to a boil. Reduce heat and simmer until potatoes are tender, about 15 minutes. Cool 30 minutes. Working in batches, puree soup in blender. Transfer soup to large bowl. Mix in half and half. Season to taste with salt and pepper. Cover and refrigerate until cold, about 4 hours. (Can be made 2 days ahead. Keep refrigerated. Before serving, thin with additional broth, if necessary.) Ladle soup into bowl. Sprinkle with chives and serve. Makes 8 servings. Bon Appétit August 2002